1) Field of the Invention
The field of this invention pertains to communications and, more particularly, to a method and apparatus for processing and detecting pulse position modulated signals.
2) Background of the Related Art
In a communication system, an information signal may be modulated onto one or more carrier signals for transmission over a communication channel. Conveniently, the amplitude, frequency, and/or time characteristics of the carrier signal or signals may be modulated by the information signal to be communicated.
One technique for modulating and transmitting digital signals is known as pulse position modulation (PPM). In a PPM communication system, a series of pulses is generated and transmitted over a communication channel. A time interval is divided into equal-length segments known as timing windows. Pulses are transmitted at an average pulse transmission rate, R, which is selected based upon the bandwidth available over the communication channel. In an exemplary PPM communication system known in the art, a single pulse is produced during each timing window. The duration of the timing window is calculated to be: EQU T.sub.timing window =1/R
Typically, the pulse width or duration is much smaller than the timing window, i.e., T.sub.pulse &lt;&lt;T.sub.timing window.
An information signal modulates the time position of the single pulse within each timing window. According to one technique, the time position of a pulse within a timing window is varied according to the amplitude of the information signal to be communicated.
In a digital communication system, the information to be transmitted generally comprises a digital bit-stream. The digital bit-stream comprises a series of bits, each bit having one of two possible amplitude levels, typically denoted as "1" and "0." Thus, in a digital communication system incorporating PPM modulation, a bit may be transmitted in each timing window, with a pulse being produced at one of two discrete time positions within each timing window, depending upon whether the bit to be transmitted corresponds to a "1" or a "0."
In many digital systems, spread spectrum communication techniques are advantageously employed for reduced transmission power requirements, efficient spectrum utilization, and improved security, including a reduced probability of detection and interception and an enhanced immunity to interference and jamming. An extensive discussion of spread spectrum communication, spreading codes, and chip sequences, may be found, e.g., in R. Dixon, SPREAD SPECTRUM SYSTEMS (J. Wiley & Sons, 2d ed. 1984).
Although PPM is a useful communication technique, one potential drawback is that because the system relies on relatively precise timing and detection of a single pulse in each timing window, it is susceptible to noise or interference which either causes false data peaks to register incorrect pulse detection in the receiver, or suppresses the actual pulse transmitted and causes a loss of information. Accordingly, it would be advantageous to provide an improved technique for processing and detecting pulse position modulated signals. It further would be advantageous to provide such a technique well adapted to the reception and detection of spread spectrum signals.